In an electronic circuit in which surface mounting type electronic components are mounted, the electronic component such as a chip resistor, a chip capacitor, or so forth is mounted exceeding over (straddling between) a plurality of electrical wirings. Hence, it is important to improve a reliability on the mounting by increasing a durability of an electrically conductive bonding agent such as a solder, a conductive resin, or so forth used in a bonding of the chip resistor or the chip capacitor to each electrical wiring. Especially, when the chip capacitor or the chip resistor is mounted on the metal substrate having a large thermal expansion rate, it is important to improve the reliability on the mounting.
Recently, in order to realize the surface mounting which enables a miniaturization and a labor saving during the mounting, various types of circuit boards are used and electronic circuits in which various types of surface mounting electronic components are mounted on such circuit boards as described above have been used. Especially, as the circuit boards on which electronic components having high heat generating characteristics are mounted, such metal substrates that insulation layers on which insulating materials such as epoxy resins or so forth are coated are disposed on the metal substrates and copper foil circuit patterns are disposed on these insulation layers have been used.
Incidentally, such an industrial demand has strongly been made that vehicle-mounted electronic equipment is to be installed within an engine compartment as well as the equipment to be miniaturized and to be saved in space.
Since the internal combustion engine is housed within the engine compartment, such a cruel environment that a temperature is high and a temperature variation is large is present. Hence, such circuit boards as being superior in a hear dissipation characteristic and as being high in a long-term reliability are required.
In circuits on the metal substrates, various kinds of electronic components are bonded via electrically conductive bonding agents such as solders, electrically conductive resins, or so forth. However, when a repetition of a temperature rise and a temperature fall in an actual use (=a heat cycle) is received over a long term, cracks in the electronically conductive bonding agents themselves or in bonding portions between the electrically conductive bonding agents and the wirings are often generated. Consequently, electrical bonding between chip resistors or chip capacitor electrodes are lost or such a problem that thermal conduction routes generated from components are interrupted is generated.
Especially, due to a thermal (heat) dissipation characteristic or an economical reason, there are many cases where aluminum plates are used as the metal substrates. It should be noted that, depending upon the situation, copper plates are sometimes used as the metal substrates. However, since a difference in a thermal expansion rate between the metal substrates and the electronic components, especially, between the metal substrates and ceramic components such as chip resistors and chip capacitors is large, the above-described problem becomes especially easy to be generated.
In order to solve the problem such that the cracks are generated in the electrically conductive bonding agents themselves such as solders or electrically conductive resins, or so forth for fixing the electronic components or in the bonding portions between the electrically conductive portions and between the electrically conductive bonding agents and the wirings, for example, in a Japanese Patent Application Publication No. 2008-072065 (Patent Document 1), such a surface mounting structure that the structure comprises: a surface mounting purpose electronic component having electrodes on both end sections of the electronic component in an elongate direction; a land connected to one of the electrodes via a solder; a wiring having a width of a side to which the wiring is connected shorter than a length of the surface mounting electronic component in a shorter direction; and a solid pattern having a width of a side to which the wiring is connected longer than a length of the surface mounting purpose electronic component has been proposed.
According to this Patent Document 1, since the surface mounting substrate is a structure in which the solid pattern is connected to the land using the wiring having a shorter width than the surface mounting purpose electronic component or the solid pattern, a stress applied to the solder(s) due to an expansion or contraction of each member such as the surface mounting electronic component and the solid pattern can be absorbed in the wiring whose width is short and the generation of the cracks due to the thermal stress on the solder(s) can be suppressed.